1. Field of the Invention
The present invention concerns the domain of underground porous media exploitation, such as the production of hydrocarbons in an oilfield, or injection of acid gases in the subsoil.
2. Description of the Prior Art
Whether it is within the context of the production of hydrocarbons or within the context of gas injection, it is essential for successful exploitation to be able to monitor fluid flow within porous media. For example, it is necessary to determine where the hydrocarbons are located within the oilfield being exploited, and then once production has started, to monitor the hydrocarbons in view of determining how to improve production. For example, if a part of the hydrocarbons remain obstructed, it may then be decided to drill other production wells or injection wells in view of improving retrieval of the hydrocarbons.
Within the context of the injection of acid gas, such as CO2, it is essential to monitor the flow of the injected gas for determining whether storage is occurring properly at the required location to evaluate the quantity that is actually stored, and to check the integrity of the reservoir to make sure there are no leaks. Using this information, it is also possible to determine whether other injection wells are necessary.
To monitor the flow of fluids that are produced or injected within porous media, there are numerous industrially-designed methods. Among such methods, the repetitive seismic method, also termed 4D seismic surveying, is used by the industry (oil or environmental). Such a method performs several seismic campaigns, at different moments in time (in general, campaigns are carried out with at least a one year interval). Thus, experts may monitor the fluid flow of the production reservoir or at the geological storage site.
Then, it is determined whether the observed variations of seismic speed (or impedance) of various acquired seismic records, at different points in time, are mainly due to variation in pressure (fluid and confinement) or due to fluid substitution of another fluid by replacing one fluid with another fluid. In other words, it is determined how fluid distributions in the reservoir vary across time.
The flow of injected or produced fluids within porous media is characterized on the basis of two sets of seismic data, via two parameters which are variation of the incompressibility module of the saturating fluid and variation of pore pressure (or fluid pressure). The first parameter indicates the type of fluid at a given point within the media and the second parameter indicates the increase of induced pressure.
These two parameters are generally determined using inversion of seismic data (speed or impedance). The Biot-Gassmann equation links the incompressibility module of a saturating fluid to seismic speed or impedance, and the Hertz-Mindlin equation links pore pressure to seismic speed or impedance
The Hertz-Mindlin equation is a power-law of the type: V=k·Ph, where V represents speed, P is pressure, k is a real number, and h is an exponent termed the Hertz coefficient. The Hertz coefficient may take on very diverse values depending on the type of rock and porosity. Values generally range from 0.005 to 0.05 for limestone, both for P and S waves, while for sandstone, values increase with porosity from 0.04 to 0.06 for P waves, and from 0.06 to 0.1 for S waves. The ratio of Hertz coefficients for S and P waves is approximately 1.6 for sandstone. This coefficient is determined using ultrasound measures (typically a few hundred kilohertz to a megahertz) for laboratory rock samples, or using acoustic survey data (typically a few kilohertz to a few dozen kilohertz). The following documents describe this procedure:    Rasolofosoan, P. N. J, Zinszner, B. E., 2006 Complete Pressure and Fluid Dependence of Seismic Properties of Reservoir Rocks. 68th EAGE Conference and Exhibition, Paper P046.    Rasolofosoan, P. N. J, Zinszner, B. E., 2004 Laboratory Peroacoustics for Seismic Monitoring Feasibility Study. The Leading Edge, v. 23; No. 3, p. 252-258.    Rasolofosoan, P. N. J, Zinszner, B. E., 2007, The Unreasonable Success of Gassmann's Theory . . . Revisited., Journal of seismic Exploitation, volume 16, Number 2-4, 281-301.
However, the frequency band of seismic data is typically from a few hertz to a few dozen hertz. The difference of frequencies between waves used to determine the law of speed/pressure, and the waves used during seismic campaigns, may be at the source of skewed interpretations, as illustrated in FIG. 3 (described later).